Optical devices, bioanalytical assays, and biological imaging probes often utilize components that exhibit optical properties, such as organic fluorophores and semi-conductor nanoparticles. Some desired optical properties include long luminescence lifetimes, large effective energy differences between excitation and emission bands, and sharp emission bands throughout the visible and near-infrared (NIR) spectral ranges. NIR optical imaging has clinical potential to significantly improve diagnosis of various human diseases in real time imaging experiments. Currently used nucleic acid-binding NIR dyes (e.g., organic fluorophores) are not fully reliable for optical imaging, in part because of low quantum yield, broad bandwith, high energy excitation wavelengths, small Stokes shift, poor water solubility, and low photostability (i.e., prone to photobleaching). These characteristics of nucleic acid-binding NIR dyes can limit detection sensitivity and deleteriously affect image resolution. While quantum dots have better resistance to photobleaching and higher quantum yields than nucleic acid-binding NIR dyes, these imaging agents may suffer from blinking (i.e., random fluctuations in light emission) and may be toxic for in vivo applications.
TABLE 1illustrates the photophysical properties of Yb3+[Zn(II)MCpyzHA] andNd3+[Zn(II)MCpyzHA] MCs in solid state and in aqueous solutions (200 μM) at roomtemperature.aτ(μs)bQ(%)dMetallacrownSolidH2OD2OqcSolidH2OD2OYb3+[Zn(II)MCpyzHA]45.6(3) 5.57(1) 81.3(1) 0  0.659(4)1.12(7) · 10−20.257(3)Nd3+[Zn(II)MCpyzHA] 1.71(1)0.214(4) 1.29(1)0.10.444(9) 7.7(1) · 10−36.17(9) · 10−2a2σ values are given between parentheses. Experimental errors: τ, ±2%, Q, ±10%.bUnder excitation at 355 nm.cThe number of coordinated water molecules (q) has been calculated using the equations:             q      Yb        =                            1                      τ            H2O                          -                  1                      τ            D2O                          -                  0.2          ⁢                                          ⁢                      (                          in              ⁢                                                          ⁢              μs                        )                    ⁢                                          ⁢          and          ⁢                                          ⁢                      q            Nd                              =                        130          ×                      (                                          1                                  τ                  H2O                                            -                              1                                  τ                  D2O                                                      )                          -        0.4              ;[274, 275] estimated error ±0.2.dUnder excitiation at 370 nm.